1. Field of the Invention
This invention relates to a colored steel sheet, especially a black sheet, used for members to be decorated and a method for making same.
2. Description of the Prior Art
Development for surface treated steel sheets of low cost and high performance has been earnestly demanded in the fields of rust proofing steel sheets for cars, steel sheets, household furnishings and building materials. These demanded standards have been yearly shifted to further lower cost and improve quality, and steel makers have developed new techniques and new products to meet the requirements of users. Recently, it has been attempted to provide precoated steel sheets instead of the conventional surface treated steel sheets which are subjected to working and then pretreatment and coating, whereby the pretreatment and coating which have been made by users can be omitted and products of high quality can be obtained at low cost. For meeting these requirements, hitherto precoated steel sheets applied with a coating composition have been used, but now demands for inorganic colored steel sheets have become greater for complete cost-down, sophisticated appearance and for improving weldability and removing damages caused by handling.
For color tone, need for blackish colors is strong and besides the above requirements, the following are required: namely, fingerprints are not left thereon; high workability, chemical resistance and corrosion resistance.
The conventional black treating method is generally applied to stainless steel, steel sheets, and copper, but since zinc plated steel sheets meet the object of this invention from the point of cost and corrosion resistance, conventional technique for blackening thereof will be explained below.
As known blackening methods of zinc or zinc alloy plated steel sheets, there are the methods of Japanese Patent KOKAI (Laid-Open) No. 45544/77 which comprises treating the sheets in an aqueous chromate solution containing silver ion (black chromate), the method disclosed in Jitsumu Hyomen Gijutsu, Vol. 32, No. 10, pages 541-542, Tables 3 and 5, and the like. According to these methods, silver oxide is coprecipitated together with a chromate film to obtain black surface. Japanese Patent KOKAI (Laid-Open) No. 65139/77 discloses a method which forms a black appearance by production of a sulfide. Japanese Patent KOKAI (Laid-Open) Nos. 151490/83 and 151491/83 disclose blackening by anode electrolysis treatment. According to the former, oxide of Ni, Co or Mo is produced by anodization in an alloy plating bath of Ni.sup.2+, Co.sup.2+ or Mo.sup.2+ and Zn.sup.2+ and according to the latter, the oxide is produced by anodization of Ni, Co or Mo and Zn alloy plated steel in an aqueous ammonium sulfate solution. Furthermore, Japanese Patent KOKAI (Laid-Open) No. 121275/85 discloses a method for obtaining black appearance by chemically dissolving an Ni-Zn alloy plated steel sheet with an aqueous solution of nitric acid or a chlorate.
Moreover, Japanese Patent KOKAI (Laid-Open) No. 200996/85 discloses a method for blackening by cathodic electrolysis in an alloy plating bath containing Zn.sup.2+ and Ni.sup.2+ to obtain a steel sheet plated with a zinc alloy containing at least 15% of Ni.
The above mentioned conventional methods are not necessarily the best ones and suffer from many problems. For example, the method using silver ion is high in cost and besides has a difficulty in productivity. The anodization method has the problem that the plated metal of the base is greatly dissolved and so the metals to be plated are limited and besides once plated metals may be redissolved.
The method of Japanese Patent KOKAI (Laid-Open) No. 200996/85 has also the problems in production that since the resulting film is an Ni-Zn alloy black plating film, there are limitations in current density and bath temperature for attaining black and substrate plating is limited to zinc or zinc alloy plating.